The period of time may be a month, quarter, or year. DSO formula: DSO = (Accounts receivable balance ÷ net credit sales) x days in period.Average days delinquent (ADD) is a metric that lets companies know the average number of days that late payments take to get collected. There are 2 calculation formulas for days in AR: The simple method and the countback method. Deliver funds to 350 South 5th Street, Room 30, Minneapolis, MN 55415, on business days between 9 a.m.